The present invention relates to a method of manufacturing compound superconductors, such as A-15, Chevrel, and B-1 type compound superconductors.
Commercially available compound superconductors are Nb.sub.3 Sn and V.sub.3 Ga. These superconductors are prepared by surface diffusion or a composite treatment which utilizes diffusion reactions between metal elements. Upper critical magnetic field of these compound superconductors are as low as about 20 tesla (T).
Examples of compound superconductors prepared on the experimental basis are Nb.sub.3 Al, Nb.sub.3 Ga, Nb.sub.3 Ge, Nb.sub.3 (AlGe) and the like of A-15 type compound superconductors, PbMo.sub.6 S.sub.8 and the like of Chevrel type compound superconductors, and NbN and the like of B-1 type compound superconductors. Among the three types of compound superconductors, Nb.sub.3 Al, PbMo.sub.6 S.sub.8 and NbN have critical temperatures of 18.9.degree. K., 14.4.degree. K. and 17.3.degree. K., and critical fields of 30 T, 50 to 60 T and 30 to 50 T, respectively. Thus, they have better superconducting properties than existing commercial available superconductors.
If the A-15, Chevrel and B-1 type compound superconductors, however, are to be prepared by surface diffusion, their particle size is increased due to the high diffusion temperature required. As a result, when a resultant superconductor is used to form a superconductor magnet, its most significant characteristic, i.e., a critical current density, is greatly reduced.
The compound superconductors can also be prepared by vapor deposition such as sputtering or CVD. According to such a method, however, the compound superconductors are prepared on only the experimental level due to a low deposition rate. Maximum dimensions of a resultant superconductor are a width of 1 cm, a length of several centimeters, and a thickness of about 2 to 3 .mu.m atmost. Although a superconductor tape having a length of several meters was reported, it had variations in superconducting characteristics along its longitudinal direction and could not be used in practical applications. According to the above conventional methods of manufacturing compound superconductors, it is difficult to manufacture a long superconductor tape. In addition, good superconductive characteristics can only be obtained if the thickness is only about 2 to 3 .mu.m. As a result, critical current Ic is as low as several 100 mA.
Niobium nitride (NbNx) can also be manufactured by thermal diffusion. An Nb body such as an Nb tape or wire is annealed in an annealing furnace in an N.sub.2 gas atmosphere to react Nb with N, thereby preparing NbN. However, a superconductor having good superconducting characteristics has an NaCl type (B-1) crystal structure, and is an NbN-.delta. phase having a 1:1 ratio of Nb to N. The .delta.-phase is a nonequilibrium phase, existing stably only at a high temperature of 1,300.degree. C. or higher. For this reason, NbN which has neither the 1:1 ratio of Nb to N nor the NaCl type crystal structure tends to be undesirably prepared. As a result, the resultant superconducting characteristics are unsatisfactory.
Sputtering is known as a method suitable for preparing an NbN nonequilibrium phase. According to this method, an Nb target and a substrate are placed in a vacuum chamber. The vacuum chamber is evacuated to a vacuum of 10.sup.-6 to 10.sup.-7 Torr. Ar and N.sub.2 gases are supplied to the vacuum chamber at a pressure of 10.sup.-1 to 10.sup.-3 Torr and RF power is supplied between the Nb plate and the substrate to generate Ar and N.sub.2 gas plasmas. Ar ions are then sputtered to remove Nb atoms from the Nb target. The Nb atoms react with N until the atoms reach the substrate, thereby forming an NbN film on the substrate. An NbN-.delta. phase film having good superconducting characteristics can thus be obtained.
With sputtering, however, using a vacuum chamber degrades productive efficiency. In addition, the shape of an NbN film formed is limited. It is difficult to form samples of a long wire or of a large area. Furthermore, since the NbN film formation rate is low, a long period of time is required.